Reaction of Cellulose in Heterogeneous Media
Due to the outstanding properties of cellulose ethers, they have a very wide range of applications and are used in a large number of technical applications and products. Despite their versatility the industrial synthesis of cellulose ethers has not changed considerably since Ernst ad Sponsel published the first industrially feasible process for cellulose ether production in the twenties of the past century.
In the following years, industrial researchers focused on improving this process by introducing different kinds of inert reaction media or by splitting the etherification into several reaction steps. All the industrial efforts were leading to processes that have several facts in common. The reaction is carried out in a heterogeneous way, i.e. cellulose and its reaction intermediates and products never leave their solid form throughout the reaction. Because of this heterogeneity the product quality strongly depends on the quality of the stirring system and on the geometry of the reaction vessel (EP 1 293 515, whose United States equivalent is U.S. Pat. No. 6,667,395).
All of the heterogeneous etherification methods described to-date require an activation of the cellulose previous to the etherification, e.g. by adding caustic soda solution to the reaction media. The caustic soda is needed for breaking the crystalline regions of the cellulose in order to facilitate the access to the etherifying agents and also as a catalyst for the reaction of epoxides and Michael substrates or as stoichiometric reactants when using halogen compounds in terms of a Williamson ether synthesis. Despite its importance for the heterogeneous reaction the alkalization of cellulose is not yet fully understood, and is under vital discussion in the academic press (C. Cuissant and P. Navard, Macromol. Symp., 2006, 244, 19). Considering the ecologic and economic aspects of the heterogeneous pathway, the application of alkali, e.g. high costs as well as energy consumption for the activation of the cellulose, purification of chemicals applied and the polymer degradation, is not preferred. Furthermore, caustic soda together with water is fed to the reaction mixture, which on one hand is needed for the activation of cellulose. On the other hand, water together with caustic soda leads to an increased formation of byproducts, i.e. glycols, alcohols and ethers, as a result of an etherification of the byproducts with unreacted etherification agent. Especially for highly etherified products the etherification efficiency is reduced due to the abundant pathways to create side products, e.g. an efficiency down to 40%. Regarding the neutralization of the reaction mixture, the alkali inevitably produces an undesirably high amount of salt load which has to be removed. Furthermore, the produced cellulose ethers exhibit a dissatisfying distribution of the substituent along and between the polymer chains. Thus, conventional cellulose ethers exhibit incomplete solubility. The regio-selective derivatizations of hydroxyl moieties of the anhydroglucose unit can not be performed in a heterogeneous way. In contrast, homogeneous production of cellulose derivatives allows the development of methods for synthesis of cellulose ethers without activation of cellulose and high yields of products with new and better properties and remedies the disadvantages of the heterogeneous synthesis.
Cellulose Derivatization in Homogeneous Media
Several solvents have been studied with regard to their application as a medium for homogeneous derivatization of cellulose under lab scale conditions. In the last two centuries, different aqueous and non-aqueous cellulose solvent systems were investigated for the etherification of cellulose in homogeneous phase. N,N-dimethylacetamide/LiCl (T. R. Dawsey, C. L. McCormick, J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1990, C30, 405) was used as reaction medium for the etherification of cellulose, e.g. methylation, hydroxyethylation, benzylation and carboxymethylation. However, this reaction medium can only be used under lab scale conditions due to bad yields, high costs and energy consumption as well as the purification of the chemicals applied. In the solvent SO2/diethylamine/dimethyl sulfoxide (A. Isogai et al., J. Appl. Polym. Sci. 1984, 29, 3873) the preparation tri-O-substituted-cellulose ethers, e.g. tri-O-arylmethylcellulose, was performed. N-methylmorpholine-N-oxide (NMMO, DE 19730090 whose United States equivalents are U.S. Pat. Nos. 6,939,960 and 6,482,940), LiCl/1,3-dimethyl-2-imidazolidinone (A. Takaragi et al., Cellulose 1999, 6, 93), N,N-dimethylformamide/N2O4 (Th. Heinze, T. Liebert, Prog. Polym. Sci. 2001, 26, 1689), Ni(tren)(OH)2 [tren=tris(2-aminoethyl)amine] aqueous solutions, melts of LiClO4×3H2O, (Th. Heinze, T. Liebert, P. Klufers, F. Meister, Cellulose 1999, 6, 153), NaOH/aqueous urea solution (J. P. Zhou et al., Macromol. Biosci. 2006, 6, 84) and the solvent dimethyl sulfoxide in combination with ammonium fluorides (S. Köhler, Th. Heinze, Macromol. Biosci. 2007, 7, 307) also have proved to be appropriate as solvents for the homogeneous preparation of unconventional cellulose derivatives. These solvent systems were not appropriate for application in larger scale due to the complicate and cost-intensive recycling of the solvent, high toxicity, volatility and limited solubility of the high molecular cellulose despite of the multiple opportunities for derivatizations of cellulose. NMMO×H2O as cellulose solvent is used for shaping (U.S. Pat. No. 4,196,282) and modification of cellulose (e.g. with acrylonitrile and methyl vinyl ketone, U.S. Pat. No. 3,447,939). The carboxymethylation of cellulose under application of sodium hydroxide (DD-PS 207380) as base is described. The etherification of cellulose with reagents with epoxy-or vinyl moieties, e.g. ethylene oxide, acrylonitrile, and with alkyl halides, e.g. methyl chloride is described in DE 19730090 in the presence of bases like sodium hydroxide. Stabilizers have to be applied due to the instability of the solvent.
The use of organic or inorganic bases and the addition of stabilizers results in degradation of the biopolymer and exhibits an enormous drawback for recycling and its application. The formation of side reactions, e.g. homolytic and heterolytic bond splitting, thermal instability or the high temperatures needed for the dissolution process, have hampered the industrial applications for etherification reactions.
Ionic Liquids
However, there is an increasing interest in new efficient and recyclable cellulose solvents. Recently, it was found that ionic liquids possess an enormous potential to dissolve cellulose. The dissolution of cellulose in liquefied N-alkyl-pyridinium or N-benzyl-pyridinium chloride salt, preferably in the presence of anhydrous nitrogen containing bases, such as pyridine, was described in 1934 (U.S. Pat. No. 1,943,176). Nowadays these salts are denominated as ionic liquids, especially room temperature ionic liquids. These molten salts typically show melting points between −100° C. and 300° C. (P. Wasserscheid, T. Welton (eds), Ionic Liquids in Synthesis 2003, WILEY-VCH, p. 1-6, 41-55 and 67-81). The solvent properties of ionic liquids can be adjusted simply by the variation of the nature of the anions and cations due to the changing polarity and size. Furthermore, ionic liquids have no measurable vapour pressure and possess thermal stability. The cellulose/ionic liquid solutions are suitable for the etherification and esterification of cellulose. WO 03/029329 (whose United States equivalents are U.S. Pat. Nos. 6,824,599 and 6,808,557) discloses a dissolution method of fibrous cellulose, wood pulp, linters, cotton balls or paper, i.e. cellulose in highly pure form, in various ionic liquids applying microwave radiation. Ionic liquids like 1-butyl-3-methylimidzolium chloride dissolve cellulose very easily without derivatization and degradation, even with a high degree of polymerization (DP) up to 6500 (O. A. El Seoud et al., Biomacromolecules, 2007, 8(9), 2629). It is already shown that ionic liquids are appropriate reaction media for the homogeneous derivatization of cellulose and even with bacterial cellulose. Thus, a large variety of acylation reactions have been described (O. A. El Seoud et al., Biomacromolecules, 2007, 8(9), 2629).
Etherification reactions of cellulose in ionic liquids are scarcely reported. In WO2005/054298 (whose United States equivalent is US Published Application 2007/0112185) the dissolution of cellulose in the IL 1-butyl-3-methylimidzolium chloride and its carboxymethylation in the presence of inorganic base, e.g. sodium hydroxide, is disclosed. The synthesized cellulose ether is subsequently separated from the solution. Dissolution and etherification are carried out in the substantial absence of water applying microwave radiation and/or pressure. KR 2006086069 discloses the steps of dissolving cellulose in an imidazolinium-based ionic liquid such as a 1-alkyl-3-alkyl-imidazolinium salt and the etherification of cellulose under homogeneous conditions using a metal hydroxide as catalyst, e.g. NaOH. Th. Heinze, K. Schwikal and S. Barthel also studied the carboxymethylation of cellulose in 1-butyl-3-methylimidazolium chloride in the presence of NaOH (Th. Heinze et al., Macromol. Biosci. 2005, 5, 520). Furthermore, the derivatization of cellulose with triphenylmethyl chloride was performed in 1-butyl-3-methylimidazolium chloride using an organic base, e.g. pyridine (T. Erdmenger et al., Macromol. Biosci., 2007, 7, 440). However, the application of organic or inorganic base encompasses severe drawbacks. Besides the polymer degradation under basic conditions, imidazolium-based ionic liquids tend to deprotonate at the C-2 position. The deprotonated imidazolium cation can be added directly to the carbonyl moiety of aldehydes (V. Aggarwal, Chem. Commun., 2002, 1612). Furthermore, incidental salts have to be removed after neutralization and pose a technical challenge. Moreover, the addition of further chemicals is cost-intensive and uneconomical due to their separation, and recycling of the ionic liquids.
A process is also known in which microcrystalline cellulose, cotton linters or Kraft cellulose are dissolved in 1-allyl-3-methyl-imidazolium chloride or 1-butyl-3-methyl-imidazolium chloride, supported by high-power ultrasound irradiation to enhance the dissolution process (J.-P. Mikkola et al., Ultrasound enhancement of cellulose processing in ionic liquids: from dissolution towards functionalization, in Green Chem. 9 [2007] 1229-1237). In the article, carboxyethylation and carboxypropylation of cellulose dissolved in the ionic liquid with 2-chloro-propanoic acid or 2-chloro-butanoic acid are briefly mentioned, without providing any details. The average degree of substitution (DS) of the thus produced cellulose ethers was rather low, except when NaOH was added to the reaction mixture. NaOH on the other hand gave rise to a degradation of the ionic liquid.
WO 2005/054298 A1 discloses a method for preparing a cellulose ether comprising mixing cellulose with an ionic liquid solvent to dissolve the cellulose, and then treating the dissolved cellulose with an etherifying agent in the presence of an inorganic base to form a cellulose ether, and subsequently separating the cellulose ether from the solution. Both the dissolution and the etherifying step are carried out in the absence of an organic base and in the substantial absence of water. The base is added in at least stoichiometric amount and must be neutralized after completion of the reaction, thereby producing a considerable amount of salt which must be washed out and discarded.
Thus, an object of the present invention was to develop a simple process for the preparation of cellulose ether which does not require the addition of any organic and/or inorganic bases and which reduces the salt load. A further object was to develop a process in which the etherification is carried out in a homogeneous reaction mixture, i.e. in which the cellulose is completely dissolved. Another object was provide a process in which a cellulose having a high degree of polymerization (DP) can be employed. A high DP in this respect means a DP of 1,000 or more, in particular 1,500 or more and even as high as 6,500.